Materials handling vehicle having a control apparatus for determining an acceleration value

ABSTRACT

A materials handling vehicle is provided comprising: a frame; wheels supported on the frame; a traction motor coupled to one of the wheels to effect rotation of the one wheel; a speed control element operable by an operator to define a speed control signal corresponding to a desired speed of the traction motor; a system associated with a steerable wheel to effect angular movement of the steerable wheel; and control apparatus coupled to the speed control element to receive the speed control signal, and coupled to the traction motor to generate a drive signal to the traction motor in response to the speed control signal to control the operation of the traction motor. The control apparatus may determine an acceleration value for the traction motor based on at least one of an angular position of the steerable wheel, a speed of the traction motor and a current position of the speed control element as defined by the speed control signal.

This application is a continuation of prior application U.S. Ser. No.12/360,353, filed Jan. 27, 2009, and entitled “A MATERIALS HANDLINGVEHICLE HAVING A CONTROL APPARATUS FOR DETERMINING AN ACCELERATIONVALUE,” which claims the benefit of: U.S. Provisional Application No.61/026,151, filed Feb. 5, 2008 and entitled “A MATERIALS HANDLINGVEHICLE HAVING A STEER SYSTEM INCLUDING A TACTILE FEEDBACK DEVICE”; U.S.Provisional Application No. 61/026,153, filed Feb. 5, 2008 and entitled“A MATERIALS HANDLING VEHICLE HAVING A CONTROL APPARATUS FOR DETERMININGAN ACCELERATION VALUE”; U.S. Provisional Application No. 61/049,158,filed Apr. 30, 2008 and entitled “A MATERIALS HANDLING VEHICLE HAVING ASTEER SYSTEM INCLUDING A TACTILE FEEDBACK DEVICE”; U.S. ProvisionalApplication No. 61/055,667, filed May 23, 2008 and entitled “A MATERIALSHANDLING VEHICLE WITH A MODULE CAPABLE OF CHANGING A STEERABLE WHEEL TOCONTROL HANDLE POSITION RATIO,” the disclosures of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a materials handling vehicle having acontrol apparatus for controlling the operation of a traction motor andmore specifically to such a vehicle having a control apparatus capableof determining a traction motor acceleration value.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,564,897 discloses a steer-by-wire system for a materialshandling vehicle. The vehicle comprises a steering tiller. The tiller,however, is not mechanically coupled to a steered wheel. A motor or anelectromagnetic brake is used to provide a counter steering resistiveforce.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, a materialshandling vehicle is provided comprising: a frame; wheels supported onthe frame; a traction motor coupled to one of the wheels to effectrotation of the one wheel; a speed control element operable by anoperator to define a speed control signal corresponding to a desiredspeed of the traction motor; a system associated with a steerable wheelto effect angular movement of the steerable wheel; and control apparatuscoupled to the speed control element to receive the speed controlsignal, and coupled to the traction motor to generate a drive signal tothe traction motor in response to the speed control signal to controlthe operation of the traction motor. The control apparatus may usepoints from one or more curves each defining an acceleration value thatvaries based on one of an angular position of the steerable wheel, thespeed of the traction motor and the speed control signal to determine anacceleration value for the traction motor.

The system may comprise a sensor generating signals indicative of anangular position of the steerable wheel.

The materials handling vehicle may further comprise a sensor associatedwith the traction motor for generating signals indicative of a speed ofthe traction motor.

In accordance with a second aspect of the present invention, a materialshandling vehicle is provided comprising: a frame; wheels supported onthe frame; a traction motor coupled to one of the wheels to effectrotation of the one wheel; a speed control element operable by anoperator to define a speed control signal corresponding to a desiredspeed of the traction motor; a system associated with a steerable wheelto effect angular movement of the steerable wheel; and control apparatuscoupled to the speed control element to receive the speed controlsignal, and coupled to the traction motor to generate a drive signal tothe traction motor in response to the speed control signal to controlthe operation of the traction motor. The control apparatus may determineacceleration values for the traction motor based on an angular positionof the steerable wheel, a speed of the traction motor and a currentposition of the speed control element as defined by the speed controlsignal.

The control apparatus may use points from curves to define theacceleration values based on the angular position of the steerablewheel, the speed of the traction motor and the current position of thespeed control element.

In accordance with a third aspect of the present invention, a materialshandling vehicle is provided comprising: a frame comprising anoperator's compartment; wheels supported on the frame; a traction motorcoupled to one of the wheels to effect rotation of the one wheel; asystem associated with a steerable wheel to effect angular movement ofthe steerable wheel about a first axis, the system comprising a controlhandle capable of being moved by an operator to define a current desiredangular position of the steerable wheel; and control apparatus varying adrive signal to the traction motor based on a steerable wheel error.

The steerable wheel error may be determined by comparing the currentdesired angular position of the steerable wheel to a current calculatedactual position of the steerable wheel.

In accordance with a fourth aspect of the present invention, a materialshandling vehicle is provided comprising: a frame comprising anoperator's compartment; wheels supported on the frame; a traction motorcoupled to one of the wheels to effect rotation of the one wheel; asystem associated with the steerable wheel to effect angular movement ofthe steerable wheel about a first axis. The system may comprise acontrol handle capable of being moved by an operator to define a desiredangular position of the steerable wheel. Further provided is a controlapparatus to vary a drive signal to the traction motor based on one ofthe desired angular position of the steerable wheel, a calculated actualposition of the steerable wheel, a steerable wheel error, and a steerrate of the control handle. The control apparatus may determine a firsttraction motor speed limit based on the desired angular position of thesteerable wheel, a second traction motor speed limit based on thecalculated actual position of the steerable wheel, a third tractionmotor speed limit based on the steerable wheel error and a fourthtraction motor speed limit based on the steer rate of the controlhandle. The control apparatus may select the smallest of the first,second, third and fourth traction motor speed limits and may use thesmallest limit when generating the drive signal to the traction motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a materials handling vehicle in whichthe present invention is incorporated;

FIG. 1A is an exploded view of a portion of an operator's compartmentincluding a floorboard from the vehicle illustrated in FIG. 1;

FIG. 2 is a schematic block diagram of a control apparatus from thevehicle illustrated in FIG. 1;

FIGS. 3-5 are perspective views of a power unit of the vehicle in FIG. 1with covers removed from the power unit;

FIG. 6 is a view of a tactile feedback device of the vehicle illustratedin FIG. 1;

FIG. 6A is a view, partially in cross section, of a pin extending downfrom a control handle base, a spring and a block fixed to a steeringcolumn plate;

FIGS. 7 and 8 are perspective views of the control handle of the vehicleillustrated in FIG. 1;

FIG. 9 is a view, partially in section, of the control handle and thetactile feedback device;

FIG. 10 illustrates a first curve C₁ used to define a steering motorspeed limit based on a current traction motor speed when the vehicle isbeing operated in a power unit first direction and a second curve C₂used to define a steering motor speed limit based on a current tractionmotor speed when the vehicle being operated in a forks first direction;

FIG. 11 illustrates a curve C₃ plotting a first traction motor speedlimit or a second traction motor speed limit as a function of a desiredsteerable wheel angular position or a calculated actual steerable wheelangular position;

FIG. 11A illustrates a curve C_(A) used to define a third traction motorspeed limit based on steerable wheel error;

FIG. 11B illustrates a curve C_(B) used to define a fourth tractionmotor speed limit based on steer rate;

FIG. 11C illustrates a curve C_(C) used to determine a firstacceleration reduction factor RF1 based on a calculated current actualangular position of the steerable wheel;

FIG. 11D illustrates a curve C_(D) used to determine a secondacceleration reduction factor RF2 based on a traction speed;

FIG. 12 illustrates a curve C₄ used to determine a first tactilefeedback device signal value based on traction motor speed;

FIG. 13 illustrates a curve C₅ used to determine a second tactilefeedback device signal value based on steerable wheel error; and

FIG. 14 illustrates in block diagram form steps for determining atactile feedback device signal setpoint TFDS.

DETAILED DESCRIPTION OF THE INVENTION

A materials handling vehicle constructed in accordance with the presentinvention, comprising a pallet truck 10 in the illustrated embodiment,is shown in FIG. 1. The truck 10 comprises a frame 20 including anoperator's compartment 30, a battery compartment 40 for housing abattery 42, a base 52 forming part of a power unit 50 and a pair of loadcarrying forks 60A and 60B. Each fork 60A, 60B comprises a correspondingload wheel assembly 62A, 62B. When the load wheel assemblies 62A, 62Bare pivoted relative to the forks 60A, 60B, the forks 60A, 60B are movedto a raised position. The operator's compartment 30 and the batterycompartment 40 move with the forks 60A, 60B relative to the power unit50.

The operator's compartment 30 is defined by an operator's backrest 32, aside wall 44 of the battery compartment 40 and a floorboard 34. Anoperator stands on the floorboard 34 when positioned within theoperator's compartment 30. In the illustrated embodiment, the floorboard34 is coupled to a frame base 20A along a first edge portion 34A viabolts 134A, washers 134B, nuts 134C, spacers 134D and flexible grommets134E, see FIG. 1A. A second edge portion 34B of the floorboard 34,located opposite to the first edge portion 34A, rests upon a pair ofsprings 135. The floorboard 34 is capable of pivoting about an axisA_(FB), which axis A_(FB) extends through the first edge portion 34A andthe flexible grommets 134E. A proximity sensor 36, see FIGS. 1A and 2,is positioned adjacent to the floorboard 34 for sensing the position ofthe floorboard 34. When an operator is standing on the floorboard 34, itpivots about the axis A_(FB) and moves towards the proximity sensor 36such that the floorboard 34 is sensed by the sensor 36. When theoperator steps off of the floorboard 34, the floorboard 34 is biased ina direction away from the sensor 36 by the springs 135 such that it isno longer sensed by the sensor 36. Hence, the proximity sensor 36generates an operator status signal indicating that either an operatoris standing on the floorboard 34 in the operator's compartment 30 or nooperator is standing on the floorboard 34 in the operator's compartment30. A change in the operator status signal indicates that an operatorhas either entered or exited the operator's compartment 30.

The power unit 50 comprises the base 52, a side wall 54 and a steeringcolumn 56, see FIGS. 3-8. The base 52, side wall 54 and steering column56 are fixed together such that the steering column 56 does not rotateor move relative to the side wall 54 or the base 52 in the illustratedembodiment. First and second caster wheels, only the first caster wheel58 is illustrated in FIG. 1, are coupled to the base 52 on opposingsides 52A and 52B of the base 52.

The power unit 50 further comprises a drive unit 70 mounted to the base52 so as to be rotatable relative to the base 52 about a first axis A₁,see FIGS. 4 and 5. The drive unit 70 comprises a support structure 71mounted to the base 52 so as to be rotatable relative to the base 52, atraction motor 72 mounted to the support structure 71, and a drivensteerable wheel 74 mounted to the support structure 71, see FIGS. 3-5.The steerable wheel 74 is coupled to the traction motor 72 so as to bedriven by the traction motor 72 about a second axis A₂, see FIG. 1. Thesteerable wheel 74 also moves together with the traction motor 72 andthe support structure 71 about the first axis A₁.

An encoder 172, see FIG. 2, is coupled to an output shaft (not shown) ofthe traction motor 72 to generate signals indicative of the speed anddirection of rotation of the traction motor 72.

The truck 10 comprises a steer-by-wire system 80 for effecting angularmovement of the steerable wheel 74 about the first axis A₁. Thesteer-by-wire system 80 comprises the control handle 90, a tactilefeedback device 100, biasing structure 110, a steer motor 120 and thesteerable wheel 74, see FIGS. 3, 4, 6 and 9. The steer-by-wire system 80does not comprise a mechanical linkage structure directly connecting thecontrol handle 90 to the steerable wheel 74 to effect steering of thewheel 74. The term “control handle” is intended to encompass the controlhandle 90 illustrated in FIG. 1 and like control handles includingsteering tillers and steering wheels.

The control handle 90 is capable of being rotated by an operatorapproximately +/−60 degrees from a centered position, wherein thecentered position corresponds to the steerable wheel 74 being located ina straight-ahead position. The control handle 90 is coupled to thetactile feedback device 100, which, in turn, is coupled to a plate 56Aof the steering column 56 via bolts 101, shown in FIG. 6 but not shownin FIG. 9. The bolts 101 pass through bores in the plate 56A and engagethreaded bores in a boss 106, shown in FIG. 9, of the tactile feedbackdevice 100. The tactile feedback device 100 may comprise an electricallycontrolled brake capable of generating a resistance or counter forcethat opposes movement of the control handle 90, wherein the force variesbased on a magnitude of a tactile feedback device signal, which signalwill be discussed below. For example, the electrically controlled brakemay comprise one of an electrorheological device, a magnetorheologicaldevice, and an electromagnetic device. In the illustrated embodiment,the tactile feedback device 100 comprises a device commerciallyavailable from the Lord Corporation under the product designation “RD2104-01.”

As illustrated in FIG. 9, the control handle 90 is fixedly coupled to ashaft 102 of the tactile feedback device 100 such that the controlhandle 90 and the shaft 102 rotate together. A magnetically controllablemedium (not shown) is provided within the device 100. A magnetic fieldgenerating element (not shown) forms part of the device 100 and iscapable of generating a variable strength magnetic field that changeswith the tactile feedback device signal. The magnetically controllablemedium may have a shear strength that changes in proportion to thestrength of the magnetic field, and provides a variable resistance orcounter force to the shaft 102, which force is transferred by the shaft102 to the control handle 90. As the variable resistance force generatedby the tactile feedback device 100 increases, the control handle 90becomes more difficult to rotate by an operator.

The tactile feedback device 100 further comprises a control handleposition sensor 100A, shown in FIG. 2 but not shown in FIG. 9, whichsenses the angular position of the control handle 90 within the angularrange of approximately +/−60 degrees in the illustrated embodiment. Thecontrol handle position sensor 100A comprises, in the illustratedembodiment, first and second potentiometers, each of which senses theangular position of the shaft 102. The second potentiometer generates aredundant position signal. Hence, only a single potentiometer isrequired to sense the angular position of the shaft 102. The angularposition of the shaft 102 corresponds to the angular position of thecontrol handle 90. An operator rotates the control handle 90 within theangular range of approximately +/−60 degrees in the illustratedembodiment to control movement of the steerable wheel 74, which wheel 74is capable of rotating approximately +/−90 degrees from a centeredposition in the illustrated embodiment. As the control handle 90 isrotated by the operator, the control handle position sensor 100A sensesthat rotation, i.e., magnitude and direction, and generates a steercontrol signal corresponding to a desired angular position of thesteerable wheel 74 to a steering control module or unit 220.

The biasing structure 110 comprises a coiled spring 112 in theillustrated embodiment, see FIGS. 6, 6A and 9, having first and secondends 112A and 112B. The spring 112 is positioned about the boss 106 ofthe tactile feedback device 100, see FIG. 9. A pin 92, shown in FIGS. 6and 6A but not shown in FIG. 9, extends down from a base 94 of thecontrol handle 90 and moves with the control handle 90. When the controlhandle 90 is located in its centered position, the pin 92 is positionedbetween and adjacent to the first and second spring ends 112A and 112B,see FIG. 6A. The spring ends 112A and 112B engage and rest against ablock 115A fixed to and extending down from the plate 56A of thesteering column 56 when the control handle 90 is in its centeredposition, see FIGS. 6 and 6A. As the control handle 90 is rotated by anoperator away from its centered position, the pin 92 engages and pushesagainst one of the spring ends 112A, 112B, causing that spring end 112A,112B to move away from the block 115A. In response, that spring end112A, 112B applies a return force against the pin 92 and, hence, to thecontrol handle 90, in a direction urging the control handle 90 to returnto its centered position. When the operator is no longer gripping andturning the control handle 90 and any resistance force generated by thetactile feedback device 100 is less than that of the biasing forceapplied by the spring 112, the spring 112 causes the control handle 90to return to its centered position.

The steering column 56 further comprises a cover portion 56B, shown onlyin FIGS. 7 and 8 and not in FIGS. 6 and 9, which covers the tactilefeedback device 100.

The steer motor 120 comprises a drive gear 122 coupled to a steer motoroutput shaft 123, see FIGS. 3 and 4. The drive unit 70 further comprisesa rotatable gear 76 coupled to the support structure 71 such thatmovement of the rotatable gear 76 effects rotation of the supportstructure 71, the traction motor 72 and the steerable wheel 74 about thefirst axis A₁, see FIGS. 3-5. A chain 124 extends about the drive gear122 and the rotatable gear 76 such that rotation of the steer motoroutput shaft 123 and drive gear 122 causes rotation of the drive unit 70and corresponding angular movement of the steerable wheel 74.

The vehicle 10 further comprises a control apparatus 200, which, in theillustrated embodiment, comprises a traction control module 210, thesteering control module 220 and a display module 230, see FIGS. 2, 3 and7. Each of the modules 210, 220 and 230 comprises a controller orprocessor for effecting functions to be discussed below. The functionseffected by the modules 210, 220 and 230 may alternatively be performedby a single module, two modules or more than three modules. The tractioncontrol module 210 is mounted to the side wall 54, the steering controlmodule 220 is mounted to the base 52 and the display module 230 ismounted within the steering column 56.

The control handle 90 further comprises first and second rotatable speedcontrol elements 96A and 96B forming part of a speed control apparatus96. One or both of the speed control elements 96A, 96B may be grippedand rotated by an operator to control a direction and speed of movementof the vehicle 10, see FIGS. 2, 7 and 8. The first and second speedcontrol elements 96A and 96B are mechanically coupled together such thatrotation of one element 96A, 96B effects rotation of the other element96B, 96A. The speed control elements 96A and 96B are spring biased to acenter neutral or home position and coupled to a signal generator SG,which, in turn, is coupled to the traction control module 210. Thesignal generator SG, for example, a potentiometer, forms part of thespeed control apparatus 96 and is capable of generating a speed controlsignal to the traction control module 210. The speed control signalvaries in sign based on the direction of rotation of the speed controlelements 96A, 96B, clockwise or counterclockwise from their homepositions, and magnitude based on the amount of rotation of the speedcontrol elements 96A, 96B from their home positions. When an operatorrotates a control element 96A, 96B in a clockwise direction, as viewedin FIG. 7, a speed control signal is generated to the traction controlmodule 210 corresponding to vehicle movement in a power unit firstdirection. When the operator rotates a control element 96A, 96B in acounter-clockwise direction, as viewed in FIG. 7, a speed control signalis generated to the traction control module 210 corresponding to vehiclemovement in a forks first direction.

The control handle 90 further comprises a speed selection switch 98, seeFIGS. 2, 7 and 8, which is capable of being toggled back and forthbetween a high speed position corresponding to a “high speed” mode and alow speed position corresponding to a “low speed” mode. Based on itsposition, the speed selection switch 98 generates a speed select signalto the traction control module 210. If the switch 98 is in its low speedposition, the traction control module 210 may limit maximum speed of thevehicle 10 to about 3.5 MPH in both a forks first direction and a powerunit first direction. If the switch 98 is in its high speed position,the traction control module 210 will allow, unless otherwise limitedbased on other vehicle conditions, see for example the discussion belowregarding FIGS. 11, 11A and 11B, the vehicle to be operated up to afirst maximum vehicle speed, e.g., 6.0 MPH, when the vehicle is beingoperated in a forks first direction and up to a second maximum vehiclespeed, e.g., 9.0 MPH, when the vehicle is being operated in a power unitfirst direction. It is noted that when an operator is operating thevehicle 10 without standing on the floorboard 34, referred to as a“walkie” mode, discussed further below, the traction control module 210will limit maximum speed of the vehicle to the maximum speedcorresponding to the switch low speed position, e.g., about 3.5 MPH,even if the switch 98 is located in its high speed position. It is notedthat the speed of the vehicle 10 within a speed range, e.g., 0-3.5 MPH,0-6.0 MPH and 0-9.0 MPH, corresponding to one of the low speedmode/walkie mode, the high speed mode/first maximum vehicle speed, andthe high speed mode/second maximum speed is proportional to the amountof rotation of a speed control element 96A, 96B being rotated.

The steer motor 120 comprises a position sensor 124, see FIG. 2. As thesteer motor output shaft 123 and drive gear 122 rotate, the positionsensor 124 generates a steer motor position signal to the steeringcontrol unit 220, which signal is indicative of an angular position ofthe steerable wheel 74 and the speed of rotation of the steerable wheel74 about the first axis A₁. The steering control unit 220 calculatesfrom the steer motor position signal a current actual angular positionof the steerable wheel 74, and the current speed of rotation of thesteerable wheel 74 about the first axis A₁. The steering control unitpasses the calculated current angular position of the steerable wheel 74and the current speed of rotation of the steerable wheel 74 to thedisplay module 230.

The steering control unit 220 also receives the steer control signalfrom the control handle position sensor 100A, which, as noted above,senses the angular position of the control handle 90 within the angularrange of approximately +/−60 degrees in the illustrated embodiment. Thesteering control unit 220 passes the steer control signal to the displaymodule 230. Since a current steer control signal corresponds to acurrent position of the control handle 90 falling within the range offrom about +/−60 degrees and the steerable wheel 74 is capable ofrotating through an angular range of +/−90 degrees, the display module230 converts the current control handle position, as indicated by thesteer control signal, to a corresponding desired angular position of thesteerable wheel 74 by multiplying the current control handle position bya ratio of equal to or about 90/60 in the illustrated embodiment, e.g.,an angular position of the control handle 90 of +60 degrees equals adesired angular position of the steerable wheel 74 of +90 degrees. Thedisplay module 230 further determines a steer rate, i.e., change inangular position of the control handle 90 per unit time, using the steercontrol signal. For example, the display module 230 may compare angularpositions of the control handle 90 determined every 32 milliseconds todetermine the steer rate.

As noted above, the proximity sensor 36 generates an operator statussignal indicating that either an operator is standing on the floorboard34 in the operator's compartment 30 or no operator is standing on thefloorboard 34 in the operators compartment 30. The proximity sensor 36is coupled to the traction control module 210 such that the tractioncontrol module 210 receives the operator status signal from theproximity sensor 36. The traction control module 210 forwards theoperator status signal to the display module 230. If an operator isstanding on the floorboard 34 in the operator's compartment 30, asindicated by the operator status signal, the display module 230 willallow movement of the steerable wheel 74 to an angular position fallingwithin a first angular range, which, in the illustrated embodiment, isequal to approximately +/−90 degrees. If, however, an operator is NOTstanding on the floorboard 34 in the operator's compartment 30, thedisplay module 230 will limit movement of the steerable wheel 74 to anangular position within a second angular range, which, in theillustrated embodiment, is equal to approximately +/−15 degrees. It isnoted that when an operator is standing on the floorboard 34 in theoperator's compartment 30, the vehicle is being operated in a ridermode, such as the high speed or the low speed mode noted above. When anoperator is NOT standing on the floorboard 34 in the operator'scompartment 30, the vehicle may be operated in the “walkie” mode, wherethe operator walks alongside the vehicle 10 while gripping andmaneuvering the control handle 90 and one of the first and secondrotatable speed control elements 96A and 96B. Hence, rotation of thesteerable wheel 74 is limited during the walkie mode to an angularposition within the second angular range.

Typically, an operator does not request that the control handle 90 beturned to an angular position greater than about +/−_(—)45 degrees fromthe centered position when the vehicle 10 is operating in the walkiemode. If a request is made to rotate the control handle 90 to an angularposition greater than about +/−45 degrees and the vehicle 10 is beingoperated in the walkie mode, the display module 230 will command thetraction control module 210 to cause the vehicle 10 to brake to a stop.If the display module 230 has caused the vehicle 10 to brake to a stop,the display module 230 will allow the traction motor 72 to rotate againto effect movement of the driven steerable wheel 74 after the controlhandle 90 has been moved to a position within a predefined range such as+/−40 degrees and the first and second speed control elements 96A and96B have been returned to their neutral/home positions.

As noted above, the steering control unit 220 passes the calculatedcurrent angular position of the steerable wheel 74 and the current speedof rotation of the steerable wheel 74 to the display module 230. Thesteering control unit 220 further passes the steer control signal to thedisplay module 230, which module 230 converts the steer control signalto a corresponding requested or desired angular position of thesteerable wheel 74. If an operator is standing on the floorboard 34 inthe operator's compartment 30, as detected by the proximity sensor 36,the display module 230 forwards the requested angular position for thesteerable wheel 74 to the steering control unit 220, which generates afirst drive signal to the steer motor 120 causing the steer motor 120 tomove the steerable wheel 74 to the requested angular position. If anoperator is NOT standing on the floorboard 34 in the operator'scompartment 30, as detected by the proximity sensor 36, the displaymodule 230 will determine if the requested angular position for thesteerable wheel 74 is within the second angular range, noted above. Ifso, the display module 230 forwards the requested angular position forthe steerable wheel 74 to the steering control unit 220, which generatesa first signal to the steer motor 120 causing the steer motor 120 tomove the steerable wheel 74 to the requested angular position. If therequested angular position for the steerable wheel 74 is NOT within thesecond angular range, the display module 230 limits the angular positionfor the steerable wheel 74 forwarded to the steering control unit 220 tothe appropriate extreme or outer limit of the second angular range. Asnoted above, the encoder 172 is coupled to the output shaft of thetraction motor 72 to generate signals indicative of the speed anddirection of rotation of the traction motor 72. The encoder signals areprovided to the traction control module 210 which determines thedirection and speed of rotation of the traction motor 72 from thosesignals. The traction control module 210 then forwards traction motorrotation speed and direction information to the display module 230. Thisinformation corresponds to the direction and speed of rotation of thesteerable wheel 74 about the second axis A₂.

The display module 230 may define an upper steering motor speed limitbased on a current traction motor speed using linear interpolationbetween points from a curve, which points may be stored in a lookuptable. When the truck 10 is being operated in a power unit firstdirection, points from a curve, such as curve C₁ illustrated in FIG. 10,may be used to define a steering motor speed limit based on a currenttraction motor speed. When the truck 10 is being operated in a forksfirst direction, points from a curve, such as curve C₂ illustrated inFIG. 10, may be used to define a steering motor speed limit based on acurrent traction motor speed. In the illustrated embodiment, thesteering motor speed upper limit decreases as the speed of the tractionmotor increases beyond about 2000 RPM, see curves C₁ and C₂ in FIG. 10.As a result, the steering motor responsiveness is purposefully slowed athigher speeds in order to prevent a “twitchy” or “overly sensitive”steering response as an operator operates the vehicle 10 at those higherspeeds. Hence, the drivability of the vehicle 10 is improved at higherspeeds. It is noted that the steering motor speed limits in curve C₂ forthe forks first direction are lower than the steering motor speed limitsin curve C₁ for the power unit first direction. An appropriate steeringmotor speed limit based on a current traction motor speed is provided bythe display module 230 to the steering control module 210. The steeringcontrol module 210 uses the steering motor speed limit when generatingthe first drive signal to the steer motor 120 so as to maintain thespeed of the steer motor 120 at a value equal to or less than thesteering motor speed limit until the steerable wheel 74 has been movedto a desired angular position. Instead of storing points from curve C₁or curve C₂, an equation or equations corresponding to each of thecurves C₁ and C₂ may be stored and used by the display module 230 todetermine a steering motor speed limit based on a current traction motorspeed.

As noted above, the steering control unit 220 passes the steer controlsignal to the display module 230, which module 230 converts the steercontrol signal to a corresponding desired angular position of thesteerable wheel 74. The steering control unit 220 also passes thecalculated current actual angular position of the steerable wheel 74 tothe display module 230. The display module 230 uses the desired angularposition for the steerable wheel 74 to determine a first upper tractionmotor speed limit using, for example, linear interpolation betweenpoints from a curve, such as curve C₃, illustrated in FIG. 11, whereinthe points may be stored in a lookup table. The display module 230further uses the calculated actual angular position for the steerablewheel 74 to determine a second upper traction motor speed limit using,for example, linear interpolation between points from the curve C₃.Instead of storing points from a curve C₃, an equation or equationscorresponding to the curve may be stored and used by the display module230 to determine the first and second traction motor speed limits basedon a desired angular position for the steerable wheel and a calculatedcurrent angular position of the steerable wheel. As is apparent fromFIG. 11, the first/second traction motor speed limit decreases as thedesired angular position/calculated angular position for the steerablewheel 74 increases so as to improve the stability of the vehicle 10during high steerable wheel angle turns.

The display module 230 compares a current desired angular position ofthe steerable wheel 74 to a current calculated actual position of thesteerable wheel 74 to determine a difference between the two equal to asteerable wheel error. Since the control handle position and thesteerable wheel position are not locked to one another, steerable wheelerror results from a delay between when an operator rotates the controlhandle 90 to effect a change in the position of the steerable wheel 74and the time it takes the steer motor 120 to effect correspondingmovement of the steerable wheel 74 to move the steerable wheel 74 to thenew angular position.

The display module 230 uses the steerable wheel error to determine athird upper traction motor speed limit using, for example, linearinterpolation between points from a curve, such as curve C_(A),illustrated in FIG. 11A, wherein the points may be stored in a lookuptable. Instead of storing points from a curve, an equation or equationscorresponding to the curve C_(A) may be stored and used by the displaymodule 230 to determine the third traction motor speed limit based onsteerable wheel error. As is apparent from FIG. 11A, the third tractionmotor speed limit generally decreases as the steerable wheel errorincreases.

The display module 230 uses the steer rate to determine a fourth uppertraction motor speed limit using, for example, linear interpolationbetween points from a curve, such as curve C_(B), illustrated in FIG.11B, wherein the points may be stored in a lookup table. Instead ofstoring points from a curve, an equation or equations corresponding tothe curve C_(B) may be stored and used by the display module 230 todetermine the fourth traction motor speed limit based on steer rate. Asis apparent from FIG. 11B, the fourth traction motor speed limitgenerally decreases as the steer rate increases.

The display module 230 determines the lowest value from among the first,second, third and fourth traction motor speed limits and forwards thelowest speed limit to the traction control module 210 for use incontrolling the speed of the traction motor 72 when generating a seconddrive signal to the traction motor 72.

The display module 230 may generate a high steerable wheel turn signalto the traction control module 210 when the steer control signalcorresponds to a steerable wheel angular position greater than about+/−7 degrees from its straight ahead position. When the display module230 is generating a high steerable wheel turn signal, the vehicle isconsidered to be in a “special for turn” mode.

In the illustrated embodiment, the traction control module 210 stores aplurality of acceleration values for the traction motor 72. Eachacceleration value defines a single, constant rate of acceleration forthe traction motor 72 and corresponds to a separate vehicle mode ofoperation. For example, a single acceleration value may be stored by thetraction control module 210 for each of the following vehicle modes ofoperation: low speed/walkie mode, forks first direction; lowspeed/walkie mode, power unit first direction; high speed mode, forksfirst direction; high speed mode, power unit first direction; specialfor turn mode, forks first direction; and special for turn mode, powerunit first direction. The traction control module 210 selects theappropriate acceleration value based on a current vehicle mode ofoperation and uses that value when generating the second drive signalfor the traction motor 72.

The display module 230 determines, in the illustrated embodiment, first,second and third acceleration reduction factors RF1, RF2 and RF3.

As noted above, the steering control unit 220 passes the calculatedcurrent actual angular position of the steerable wheel 74 and thecurrent speed of rotation of the steerable wheel 74 to the displaymodule 230. The display module 230 may use the calculated current actualangular position of the steerable wheel 74 to determine the firstacceleration reduction factor RF1 using, for example, linearinterpolation between points from a curve, such as curve C_(C),illustrated in FIG. 11C, wherein the points may be stored in a lookuptable. Instead of storing points from a curve, an equation or equationscorresponding to the curve C_(C) may be stored and used by the displaymodule 230 to determine the first acceleration reduction factor RF1. Asis apparent from FIG. 11C, after a steered wheel angle of about 10degrees, the first acceleration reduction factor RF1 decreases generallylinear as the steerable wheel angle increases.

As discussed above, the traction control module 210 forwards tractionmotor rotation speed and direction information to the display module230. The display module 230 may use the traction motor speed todetermine the second acceleration reduction factor RF2 using, forexample, linear interpolation between points from a curve, such as curveC_(D), illustrated in FIG. 11D, wherein the points may be stored in alookup table. Instead of storing points from a curve, an equation orequations corresponding to the curve C_(D) may be stored and used by thedisplay module 230 to determine the second acceleration reduction factorRF2. As is apparent from FIG. 11D, the second acceleration reductionfactor RF2 generally increases as the traction motor speed increases.

As noted above, an operator may rotate one or both of the first andsecond speed control elements 96A, 96B causing the signal generator SGto generate a corresponding speed control signal to the traction controlmodule 210. The traction control module 210 forwards the speed controlsignal to the display module 230. As also noted above, the speed controlsignal varies in magnitude based on the amount of rotation of the speedcontrol elements 96A, 96B from their home positions. Hence, the speedcontrol signal is indicative of the current position of the speedcontrol elements 96A, 96B. The display module 230 may determined thethird acceleration reduction factor RF3 using the speed control signal.For example, the third acceleration reduction factor RF3 may equal afirst predefined value, e.g., 10, for all speed control signalscorresponding to a position of each speed control element 96A, 96Bbetween a zero or home position and a position corresponding to 80% ofits maximum rotated position and may equal a second predefined value,e.g., 128, for all speed control signals corresponding to a position ofeach speed control element 96A, 96B greater than 80% of its maximumrotated position.

The display module 230 determines which of the first, second and thirdreduction factors RF1, RF2 and RF3 has the lowest value and providesthat reduction factor to the traction control module 210. The tractioncontrol module 210 receives the selected reduction factor, which, in theillustrated embodiment, has a value between 0 and 128. The module 210divides the reduction factor by 128 to determine a modified reductionfactor. The modified reduction factor is multiplied by the selectedacceleration value to determine an updated selected acceleration value,which is used by the traction control module 210 when generating thesecond drive signal to the traction motor 72. The reduction factorhaving the lowest value, prior to being divided by 128, effects thegreatest reduction in the acceleration value.

Based on the position of the speed selection switch 98, the operatorstatus signal, whether a high steerable wheel turn signal has beengenerated by the display module 230, the sign and magnitude of a speedcontrol signal generated by the signal generator SG in response tooperation of the first and second rotatable speed control elements 96Aand 96B, an acceleration value corresponding to the current vehicle modeof operation, a selected acceleration reduction factor, a currenttraction motor speed and direction as detected by the encoder 172, and aselected traction motor speed limit, the traction control module 210generates the second drive signal to the traction motor 72 so as tocontrol the speed, acceleration and direction of rotation of thetraction motor 72 and, hence, the speed, acceleration and direction ofrotation of the steerable wheel 74 about the second axis A₂.

Instead of determining first, second and third reduction factors,selecting a lowest reduction factor, dividing the selected reductionfactor by 128 and multiplying the modified reduction factor by aselected acceleration value to determine an updated selectedacceleration value, the following steps may be implemented by thedisplay module 230 either alone or in combination with the tractioncontrol module 210. Three separate curves are defined for each vehiclemode of operation, which modes of operation are listed above. The firstcurve defines a first acceleration value that varies based on thecalculated current actual angular position of the steerable wheel 74.The second curve defines a second acceleration value that varies basedon traction motor speed. The third curve defines a third accelerationvalue that varies based on the speed control signal from the signalgenerator SG. The display module and/or the traction control moduledetermines using, for example, linear interpolation between points fromeach of the first, second and third curves corresponding to the currentvehicle mode of operations, wherein the points may be stored in lookuptables, first, second and third acceleration values, selects the lowestacceleration value and uses that value when generating the second drivesignal to the traction motor 72.

As noted above, the tactile feedback device 100 is capable of generatinga resistance or counter force that opposes movement of the controlhandle 90, wherein the force varies based on the magnitude of thetactile feedback device signal. In the illustrated embodiment, thedisplay module 230 defines a setpoint TFDS for the tactile feedbackdevice signal, communicates the setpoint TFDS to the steering controlmodule 220 and the steering control module 220 generates a correspondingtactile feedback device signal, e.g., a current measured for example inmilliAmperes (mA), to the tactile feedback device 100.

In the illustrated embodiment, the display module 230 defines thetactile feedback device signal setpoint TFDS as follows. The displaymodule 230 constantly queries the traction control module 210 for speedand direction of rotation of the traction motor 72, which information isdetermined by the traction control module 210 from signals output by theencoder 172, as noted above. Based on the traction motor speed, thedisplay module 230 determines a first tactile feedback device signalvalue TFD1, see step 302 in FIG. 14, using, for example, linearinterpolation between points from a curve, such as curve C₄, illustratedin FIG. 12, wherein the points may be stored in a lookup table. Insteadof storing points from a curve, an equation or equations correspondingto the curve C₄ may be stored and used by the display module 230 todetermine the first value TFD1. As can be seen from FIG. 12, the firstvalue TFD1 generally increases with traction motor speed.

As noted above, the display module 230 compares the current desiredangular position of the steerable wheel 74 to a current calculatedactual position of the steerable wheel 74 to determine a differencebetween the two equal to a steerable wheel error. Based on the steerablewheel error, the display module 230 determines a second tactile feedbackdevice signal value TFD2, see step 302 in FIG. 14, using, for example,linear interpolation between points from a curve, such as curve C₅,illustrated in FIG. 13, wherein the points may be stored in a lookuptable. Instead of storing points from a curve, an equation or equationscorresponding to the curve C₅ may be stored and used by the displaymodule 230 to determine the second value TFD2. As can be seen from FIG.13, the second value TFD2 generally increases with steerable wheelerror.

In the illustrated embodiment, the display module 230 sums the first andsecond values TFD1 and TFD2 together to determine a combined tactilefeedback device signal value TFDC, see step 304 in FIG. 14, andmultiplies this value by a reduction factor based on a direction inwhich the vehicle 10 is moving in order to determine the tactilefeedback device signal setpoint TFDS, see step 306 in FIG. 14. If thevehicle 10 is being driven in a forks first direction, the reductionfactor may equal 0.5. If the vehicle 10 is being driven in a power unitfirst direction, the reduction factor may equal 1.0. Generally, anoperator has only one hand on the control handle 90 when the vehicle 10is moving in the forks first direction. Hence, the reduction factor of0.5 makes it easier for the operator to rotate the control handle 90when the vehicle 10 is traveling in the forks first direction.

The display module 230 provides the tactile feedback device signalsetpoint TFDS to the steering control unit 220, which uses the setpointTFDS to determine a corresponding tactile feedback device signal for thetactile feedback device 100.

Because the tactile feedback device signal is determined in theillustrated embodiment from the first and second values TFD1 and TFD2,which values come from curves C₄ and C₅ in FIGS. 12 and 13, the tactilefeedback device signal increases in magnitude as the traction motorspeed and steerable wheel error increase. Hence, as the traction motorspeed increases and the steerable wheel error increases, the counterforce generated by the tactile feedback device 100 and applied to thecontrol handle 90 increases, thus, making it more difficult for anoperator to turn the control handle 90. It is believed to beadvantageous to increase the counter force generated by the tactilefeedback device 100 as the traction motor speed increases to reduce thelikelihood that unintended motion will be imparted to the control handle90 by an operator as the vehicle 10 travels over bumps or into holes/lowspots found in a floor upon which it is driven and enhance operatorstability during operation of the vehicle. It is further believed to beadvantageous to increase the counter force generated by the tactilefeedback device 100 as the steerable wheel error increases so as toprovide tactile feedback to the operator related to the magnitude of thesteerable wheel error.

In a further embodiment, a pressure transducer 400, shown in dotted linein FIG. 2, is provided as part of a hydraulic system (not shown) coupledto the forks 60A and 60B for elevating the forks 60A and 60B. Thepressure transducer 400 generates a signal indicative of the weight ofany load on the forks 60A and 60B to the display module 230. Based onthe fork load, the display module 230 may determine a third tactilefeedback device signal value TFD3 using, for example, linearinterpolation between points from a curve (not shown), where the valueTFD3 may vary linearly with fork load such that the value TFD3 mayincrease as the weight on the forks 60A and 60B increases. The displaymodule 230 may sum the first, second and third values TFD1, TFD2 andTFD3 together to determine a combined tactile feedback device signalvalue TFDC, which may be multiplied by a reduction factor, noted above,based on a direction in which the vehicle 10 is moving in order todetermine a tactile feedback device signal setpoint TFDS. The displaymodule 230 provides the tactile feedback device signal setpoint TFDS tothe steering control unit 220, which uses the setpoint TFDS to determinea corresponding tactile feedback device signal for the tactile feedbackdevice 100.

As discussed above, the proximity sensor 36 outputs an operator statussignal to the traction control module 210, wherein a change in theoperator status signal indicates that an operator has either steppedonto or stepped off of the floorboard 34 in the operator's compartment30. As also noted above, the traction control module 210 provides theoperator status signal to the display module 230. The display module 230monitors the operator status signal and determines whether an operatorstatus signal change corresponds to an operator stepping onto orstepping off of the floorboard 34. An operator stops the vehicle beforestepping out of the operator's compartment. When the operator leaves theoperator's compartment, if the tactile feedback device signal is at aforce generating value, e.g., a non-zero value in the illustratedembodiment, causing the tactile feedback device 100 to generate acounter force to the control handle 90, the display module 230 decreasesthe tactile feedback device signal setpoint TFDS at a controlled rate,e.g., 900 mA/second, until the tactile feedback device signal setpointTFDS, and, hence, the tactile feedback device signal, equal zero. Byslowly decreasing the tactile feedback device signal setpoint TFDS and,hence, the tactile feedback device signal, at a controlled rate andpresuming the control handle 90 is positioned away from its centeredposition, the biasing structure 110 is permitted to return the controlhandle 90 back to its centered position, i.e., 0 degrees, withoutsubstantially overshooting the centered position after the operator hasstepped off the floorboard 34. The tactile feedback device signalsetpoint TFDS, and, hence, the tactile feedback device signal, aremaintained at a zero value for a predefined period of time, e.g., twoseconds. Thereafter, the display module 230 determines an updatedtactile feedback device signal setpoint TFDS and provides the updatedtactile feedback device signal setpoint TFDS to the steering controlunit 220. It is contemplated that the display module 230 may onlydecrease the tactile feedback device signal setpoint TFDS if, inaddition to an operator leaving the operator's compartment and thetactile feedback device signal being at a force generating value, thecontrol handle 90 is positioned away from its centered position. It isfurther contemplated that the display module 230 may maintain thetactile feedback device signal setpoint TFDS at a zero value until itdetermines that the control handle 90 has returned to its centeredposition.

If, while monitoring the operator status signal, the display module 230determines that an operator status signal change corresponds to anoperator stepping onto the floorboard 34, the display module 230 willimmediately increase the tactile feedback device signal setpoint TFDSfor a predefined period of time, e.g., two seconds, causing acorresponding increase in the tactile feedback device signal. Theincrease in the tactile feedback signal is sufficient such that thetactile feedback device 100 generates a counter force of sufficientmagnitude to the control handle 90 to inhibit an operator from making aquick turn request via the control handle 90 just after the operator hasstepping into the operator's compartment 30. After the predefined timeperiod has expired, the display module 230 determines an updated tactilefeedback device signal setpoint TFDS and provides the updated tactilefeedback device signal setpoint TFDS to the steering control unit 220.

Also in response to determining that an operator has just stepped ontothe floorboard 34 and if a steer request is immediately made by anoperator via the control handle 90, the display module 230 provides aninstruction to the steering control module 220 to operate the steermotor 120 at a first low speed, e.g., 500 RPM and, thereafter, ramp upthe steer motor speed, e.g., linearly, to a second higher speed over apredefined period of time, e.g., one second. The second speed is definedby curve C₁ or curve C₂ in FIG. 10 based on a current traction motorspeed. Hence, the first drive signal to the steer motor 120 is variedsuch that the speed of the steer motor 120, i.e., the rate of speedincrease, gradually increases from a low value after the operator entersthe operator's compartment in order to avoid a sudden sharp turnmaneuver.

It is further contemplated that the steerable wheel may not be driven.Instead, a different wheel forming part of the vehicle would be drivenby the traction motor 72. In such an embodiment, the traction controlmodule 210 may generate a second drive signal to the traction motor 72so as to control the speed, acceleration and direction of rotation ofthe traction motor 72 and, hence, the speed, acceleration and directionof rotation of the driven wheel based on the position of the speedselection switch 98, the operator status signal, whether a highsteerable wheel turn signal has been generated by the display module230, the sign and magnitude of a speed control signal generated by thesignal generator SG in response to operation of the first and secondrotatable speed control elements 96A and 96B, an acceleration valuecorresponding to the current vehicle mode of operation, a selectedacceleration reduction factor, a current traction motor speed anddirection as detected by the encoder 172, and a selected traction motorspeed limit.

It is still further contemplated that a vehicle including a mechanicalor hydrostatic steering system may include a traction motor 72controlled via a traction control module 210 and a display module 230 asset out herein presuming the vehicle includes a control handle positionsensor or like sensor for generating signals indicative of an angularposition of the control handle and its steer rate and a position sensoror like sensor for generating signals indicative of an angular positionof a steerable wheel and a speed of rotation of the steerable wheelabout an axis A₁.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed is:
 1. A materials handling vehicle comprising: a frame;wheels supported on said frame; a traction motor coupled to one of saidwheels to effect rotation of said one wheel; a speed control elementoperable by an operator to define a speed control signal correspondingto a desired speed of said traction motor; a system associated with asteerable wheel to effect angular movement of said steerable wheel;control apparatus coupled to said speed control element to receive saidspeed control signal, and coupled to said traction motor to generate adrive signal to said traction motor in response to said speed controlsignal to control the operation of said traction motor; said controlapparatus uses points from one or more curves each defining anacceleration value that varies based on one of an angular position ofsaid steerable wheel, the speed of said traction motor and the speedcontrol signal to determine an acceleration value for said tractionmotor.
 2. The materials handling vehicle as set out in claim 1, whereinsaid system comprises a sensor generating signals indicative of anangular position of said steerable wheel.
 3. The materials handlingvehicle as set out in claim 2, further comprising a sensor associatedwith said traction motor for generating signals indicative of a speed ofsaid traction motor.
 4. A materials handling vehicle comprising: aframe; wheels supported on said frame; a traction motor coupled to oneof said wheels to effect rotation of said one wheel; a speed controlelement operable by an operator to define a speed control signalcorresponding to a desired speed of said traction motor; a systemassociated with a steerable wheel to effect angular movement of saidsteerable wheel; control apparatus coupled to said speed control elementto receive said speed control signal, and coupled to said traction motorto generate a drive signal to said traction motor in response to saidspeed control signal to control the operation of said traction motor;said control apparatus determining acceleration values for said tractionmotor based on an angular position of said steerable wheel, a speed ofsaid traction motor and a current position of said speed control elementas defined by said speed control signal.
 5. The materials handlingvehicle as set out in claim 4, wherein said system comprises a sensorgenerating signals indicative of an angular position of said steerablewheel.
 6. The materials handling vehicle as set out in claim 5, furthercomprising a sensor associated with said traction motor for generatingsignals indicative of a speed of said traction motor.
 7. The materialshandling vehicle as set out in claim 4, wherein said control apparatususes points from curves to define said acceleration values based on theangular position of said steerable wheel, the speed of said tractionmotor and the current position of said speed control element.
 8. Amaterials handling vehicle comprising: a frame comprising an operator'scompartment; wheels supported on said frame; a traction motor coupled toone of said wheels to effect rotation of said one wheel; a systemassociated with a steerable wheel to effect angular movement of saidsteerable wheel about a first axis, said system comprising a controlhandle capable of being moved by an operator to define a current desiredangular position of said steerable wheel; control apparatus varying adrive signal to said traction motor based on a steerable wheel error. 9.The materials handling vehicle as set out in claim 8, wherein saidsteerable wheel error is determined by comparing said current desiredangular position of said steerable wheel to a current calculated actualposition of said steerable wheel.
 10. A materials handling vehiclecomprising: a frame comprising an operator's compartment; wheelssupported on said frame; a traction motor coupled to one of said wheelsto effect rotation of said one wheel; a system associated with saidsteerable wheel to effect angular movement of said steerable wheel abouta first axis, said system comprising a control handle capable of beingmoved by an operator to define a desired angular position of saidsteerable wheel; control apparatus varying a drive signal to saidtraction motor based on one of said desired angular position of saidsteerable wheel, a calculated actual position of said steerable wheel, asteerable wheel error, and a steer rate of said control handle, whereinsaid control apparatus determines a first traction motor speed limitbased on said desired angular position of said steerable wheel, a secondtraction motor speed limit based on said calculated actual position ofsaid steerable wheel, a third traction motor speed limit based on saidsteerable wheel error and a fourth traction motor speed limit based onsaid steer rate of said control handle, said control apparatus selectsthe smallest of the first, second, third and fourth traction motor speedlimits and uses said smallest limit when generating said drive signal tosaid traction motor.